This study examines the relationship between in vitro markers of platelet activity and aspirin dose, further correlating platelet markers with patient features and concomitant medication use. It will be the largest, most methodologically-sound survey to date. Its main objectives are to determine if the aspirin dose differentially affects markers of platelet activity in elderly people with vascular disease; to determine if patient features are related to markers of platelet activity in elderly people with vascular disease; and to assess the effect of different aspirin doses on blood pressure.